1. Field of the Invention
This invention relates to radar, and more particularly to radar transceivers for short-range target detection.
2. Description of the Prior Art
Frequency modulated continuous wave (FMCW) radar systems using solid state components are commonly found in short-range radar detection applications. Prior art FMCW radar transceivers have been based on microwave power sources such as a vacuum tube, typically a kylstron, or a transferred electron device such as a Gunn diode. These conventional approaches do not lend themselves to monolithic circuit fabrication techniques which are ideal for high production implementation.
In U.S. Pat. No. 4,742,354to Cheng P. Wen, et al., assigned to the assignee of this invention, the present applicant describes a radar transceiver employing a transmitter oscillator, a directional coupler for splitting the oscillator signal into two signals in quadrature phase relationship, and a single antenna for radiating the signals as a circularly polarized waveform. Portions of the circularly polarized waveform that are reflected back are received by the same antenna. The directional coupler combines the two components of the circularly polarized wave into a signal which is down-converted into a signal of intermediate frequency by a mixer. The radar transceiver is hybrid integrated and is fabricated on a single substrate such as a Duroid substrate. A voltage controlled Gunn oscillator is suggested as the transmitter oscillator which, as noted above, does not lend itself to monolithic circuit fabrication techniques. Specific fabrication techniques are not described with respect to the oscillator.
Background art described in this patent includes:
(1) radar transceivers having separate transmitter and receiver antennas; PA1 (2) those having a single antenna in which the transmitted and returned signals are directed through a circulator; and PA1 (3) those having a single antenna employing a 3-dB coupler for signal coupling at the antenna.
All such systems, as presently known, typically include conventional oscillators such as a klystron or a Gunn oscillator. A FET oscillator such as a GaAs FET oscillator is not employed.
In fact, in a paper entitled "Solid State Local Oscillator Sources for Millimeter and Sub-Millimeter Wave Applications", by Terrial Cutsinger, presented at the 12th International Conference on Infrared and Millimeter Waves, Dec. 14-18, 1987, Orlando, Fla., it was stated:
"Solid state devices such as FET's and IMPATT dodes are capable of generating power at millimeter-wave frequencies. Both of these however, have inherent problems which limit their usefulness. FET oscillators provide relatively low power and produce high 1/F noise. Additionally, they are limited to the lower millimeter-wave frequencies, but IMPATTS exhibit high AM noise which degrades system noise performance."
In this paper, Gunn device oscillators were stated to provide a reliable solution to the problems inherent to FET and IMPATT oscillators, pointing to their sufficiency of RF power, low AM and FM noise, and the ability to drive a mixer up to 110 GHz without frequency multiplication.